1. Field of the Invention
This invention concerns a vacuum brazing sheet for use in drawn-cup type heat exchangers made of aluminum and, more specifically, it relates to a vacuum brazing sheet capable of obtaining excellent brazing property upon assembling a drawn-cup type heat exchanger made of aluminum having the degree of closure from 0.2 to 4.0 cm.sup.2 /l manufactured by a vacuum brazing method.
2. Description of the Prior Art
A drawn-cup heat exchanger has a structure as shown in FIG. 5(b) in which tubular members 11 fabricated from a brazing sheet into a cup-shape are stacked and fins 12 are arranged between the tubular members 11 and the inside thereof is semi-sealed. Accordingly, the brazing joined portion comprises, as shown in FIG. 5(a), a joint portion situated at the outer surface 13 and a joined portion situated at an inner surface 14, both of which should be joined sufficiently by brazing.
Further, a vacuum brazing method without requiring the use of fluxes has been developed as the brazing process and employed generally since the method is free from the problems of environmental pollutions. The vacuum brazing method is mainly used also in the brazing of the drawn-cup heat exchanger.
The brazing sheet for use in the vacuum brazing of the drawn-cup heat exchanger employed generally so far is made JIS-BA8PC material comprising:
brazing material: Al-10Si-1.5 Mg PA1 core material: 3003 PA1 Si 11-14 wt %, PA1 Mg 1.0-2.0 wt %, and
While the brazing performance of the material at the outer surface of the drawn-cup heat exchanger is generally satisfactory, the brazing performance at the inner surface is not sufficient in view of the fillet-forming thus some time failing to obtain a predetermined joining strength, which causes a reduction in the yield.
As the countermeasure, there has been proposed to:
(1) reduce the Mg content in the brazing composition at the brazing surface of the inner side (refer to Japanese patent publication No. 54909/1983), or
(2) incorporate Bi, Sn, Sb, Ba, Be, etc. to the brazing composition at the brazing surface of the inner side (refer to Japanese patent laid-open No. 85375/1984).
However, since the inner and the outer surfaces lie on the identical surface of brazing material in the drawn-cup heat exchanger, they can not be actually applied and, accordingly, there have not yet been found satisfactory improving methods.